When discussing RF coaxial cable assemblies we frequently hear, "What is the RF stability of the assembly?" The trouble is, depending on the context of the application, a variety of answers may apply. This blog is intended to illustrate the value of employing a more conservative methodology. One that closely aligns with published International Electrotechnical Commission (IEC) documents, but applies environmental conditional logic to arrive at test procedures that will absolutely ensure performance in the field. It is important to realize however, that the evolution of test protocols that originated within the US Military procurement agencies (MIL-STD) and were adopted and updated by the IEC, may still develop into more improved practices over time. Comparing and contrasting our own San-tron, Inc. (STI) test procedures with those published by the IEC, we hope to demonstrate how a different perspective and approach nets optimal results in the field today.
In the process of implementation of test procedures, it is essential to first identify the true intent of the characterization itself. One intent may be to characterize the limiting performance characteristics of the assembly. As a result, the test protocol will require manipulation of the cable to its actual limiting parameters (as opposed to those outlined by the "standard"), for example: dynamic minimum bend radius, maximum torque specification, and operational thermal limits of the components. Another intent may be to characterize the cable performance within the scope of an operational deployment such as avionics, outdoor cellular communications, or test & measurement. In so doing, it becomes easier to compare alternative solutions and how they will support the intended application.
Test & Measurement Deployments
For the purpose of this discussion, we'll demonstrate the characterization of test & measurement deployments of test port cables in the use of Vector Network Analyzers (VNA) and Passive Intermodulation (PIM) testing. The typical test port cable to be validated will range in length from 40" (1m) to 13' (4m). Access to the test device, the DUT, is varied; it can be simply located on a test bench, within an environmental chamber, a rack mounted sub-system, tower mounted system, or an aircraft installation. In all of these applications a test cable will be subjected to repeated bending and twisting. Therefore, we would like to make the generalization that the cable deployment can be characterized by three basic activities: folding and twisting the cable about a mandrel and low frequency vibration. Regardless of the cable size, it will be subjected to these very realistic stresses and strains.
To illustrate these concepts we are using product data from the San-tron SRXTM series of assembly solutions, for both system integration and test and measurement. The most widely used product within this offering is the SRX141TM coaxial cables with eSeriesTM connector terminations. We will use this cable as an example of stability characterization.
For context, allow us to look back a couple years. The original prototype for this cable was an FEP jacketed cable with Ag clad Cu wire braid, Ag clad Cu foil helix, PTFE dielectric, and Ag clad solid Cu center conductor. Utilizing the cable fold test outlined in the standard for STI 8.2.4-14 A1 the prototype functioned very well. However, during the cable twist test, STI 8.2.4-14 B1, the prototype showed probable instabilities in both insertion loss and PIM performance, this required a closer look. Upon applying the vibration test of STI 8.2.4-14 C2 we recognized catastrophic degradation of the cable performance over time. The root cause of this degradation was that the physical makeup of the cable had the ability to loosen under the weave of the braid and thus generate PIM signals and exhibit unstable insertion loss performance. Therefore a design review and process upgrade was required for this product line. Ultimately we developed a cable structure that would maintain its performance through harsh environments and test protocols.
The sample test cable is coded as an "SRX141/0401/0401/60 PIM." This identifies an SRX141TM cable assembly, 60" long, terminated with straight eSeriesTM type-N male connectors, and is rated for PIM applications. The NWA is setup with 1 test port cable, a full 2-port calibration via a type-N calibration kit, 10 MHz to 10 GHz, averaging set to 6, with displays of VSWR, S21 log magnitude, and S21 phase. After calibration the test port cable is secured with a 10-lb table vice to help prevent test data variation. The DUT is mated to the NWA laid out as a straight U-shape. Both of the S21 displays are normalized with the "Data/Mem" math function. The cable assembly is then first subjected to the Cable Fold Test, 5-cycles, and then to the Cable Twist Test, 5-cycles. The worst case results are recorded as follows:
A Closer Look at the Phase Data
Let's take a closer look at the phase data. The initial data, pictured to the right, establishes the zero line within -0.30 /+0.03 degrees. Folding the sample cable assembly 5 repeated cycles in both directions about the 5" diameter mandrel results in phase shifts with the tabulated extreme readings.
The clockwise folding of the sample, pictured on the left below, results in -0.16 degrees/GHz increase in electrical length. The spring back from unfolding the sample results in a +0.08 degrees/GHz decrease in electrical length; a 0.11 degrees/GHz offset from the initial condition. The counter-clockwise folding of the sample results in -0.09 degrees/GHz expansion in electrical length.
The spring back from unfolding the sample, pictured on the right below, results in +0.26 degrees/GHz decrease in electrical length; a 0.29 degrees/GHz offset from the initial condition. The resultant phase shift due to randomly folding the sample could express itself anywhere asymmetrically between -0.16 to +0.26 degrees/GHz. Therefore, we will consider the phase stability, as a function of folding the sample, to be 0.42 degrees/GHz.
Traditionally the cable twist test generates a greater level of performance degradation and is therefore applied after the cable fold test. By now applying a positive 180 degree physical twist of the sample we see a phase shift of +0.54 degrees/GHz decrease in electrical length. Relaxing the twist back to the neutral position results in a +0.71 degrees/GHz shift. Then applying a negative 180 degree physical twist of the sample we see a phase shift of +0.45 degrees/GHz; the electrical length expanded. And then relaxing the twist back to the neutral position results in a +0.74 degrees/GHz shift decrease in electrical length.
These 2 test results illustrate a phase shift of (-0.16 / +0.26) & (+0.45/+0.74) degrees / GHz. We may combine these results (-0.16 / +0.74) to publish a range of 0.90 degrees/GHz phase stability. Also notice that when the cables are relaxed the phase shift does not necessarily revert back to zero degrees; we see bounce back to be anywhere between +0.08 to +0.74 degrees/GHz.
As we can see there are multiple mechanisms that generate instability. It is apparent that twisting a cable develops very different results than simply folding the cable. We also see that the effect of twisting decreases the electrical length whereas folding can have both an elongation and a constriction effect upon phase length. By modeling the intended application and applying tests in a progressive protocol we can develop a test parameter and degrees/GHz that helps establish the measurement uncertainty in which the product will perform in the field.
Our Cable Assembly Test Methods
Our STI methods are a composite of US Military and IEC test protocols. The main goal was to establish quantifiable characteristics for insertion loss stability, phase stability, and PIM stability. There are three basic STI test protocols to identify these performance levels:
- Cable Fold Test
- Twist Test
- Vibration Test
To learn more about PIM applications, read our blog post, "What's all this PIM stuff anyways?"